IL-1 (interleukin-1) is the cytokine that the body produces, in response to infections, various kinds of attack or antigenic stimulation, to initiate a defence reaction of the inflammatory or immune type. IL-1 is a polypeptide of approx. 17.5 kDa in its mature form, produced mainly by the macrophages but also by epidermal, lymphoid, vascular and epithelial cells. IL-1 is one of the principal stimulating factors of both the inflammatory and immune responses and, in its circulating form, it is capable of acting as a hormone, inducing a broad spectrum of systemic changes at metabolic, neurological, haematological and endocrinological level. Thus, IL-1 exerts an influence on mesenchymal tissue remodelling, contributing both to destructive processes and to repair processes. Furthermore, IL-1 is an activator of lymphocytes and plays a fundamental role in the initiation and amplification of the immune response. IL-1 also possesses strong activity of the inflammatory type, for example stimulation of the production of prostanoids and of proteases in various cells, including chondrocytes, fibroblasts, synovial cells, and brain cells. Thus, IL-1 is involved in many components of the acute-phase response and is the endogenous mediator of fever (endogenous pyrogen). IL-1 can act in synergy with other cytokines, especially TNF.alpha., significantly amplifying its inflammatory activity.
Cloning of IL-1 has led to the identification of two active forms. The predominant form is IL-1.beta., synthesized as an inactive precursor of 269 amino acids (31 kDa), which is then cut by a protease to give rise to the active mature form (corresponding to amino acids 117-269 of the precursor). A form that occurs about a hundred times less frequently, and is generally associated with the cells, is IL1.alpha., which has about 26% homology with IL1.beta., and which is also synthesized as a precursor of 271 amino acids (which, however, possesses biological activity), which then gives rise to the mature form after proteolysis of the precursor. IL-1 represents a special case among the cytokines (together with fibroblast growth factor=FGF) in that it lacks a signal peptide and so is not secreted via the normal routes. The most abundant extracellular form therefore consists of the mature form of IL1.beta., which is thus responsible for the majority of the biological activities of IL-1, both immunostimulant and inflammatory.
In view of such activities, it has been hypothesized that IL-1 might have a role in the pathogenesis of inflammatory and autoimmune diseases. Thus, in the vast majority of pathologies of acute and chronic inflammation and in many autoimmune pathologies, increased production of IL-1.beta. has been identified as one of the main factors responsible for the pathology (Dinarello C.A. Blood 77: 1, 1991).
The biological activities of IL-1 are inhibited in the presence of specific inhibitors. In view of IL-1's fundamental role in the pathogenesis of many autoimmune diseases and of chronic inflammatory diseases with tissue destruction, it is suggested that inhibition of IL-1 could be useful in the treatment of these pathologies. IL-1ra (IL-1 receptor antagonist) is a cytokine that is structurally very similar to IL-1, but is synthesized with a signal peptide and secreted as mature glycosylated protein. A non-glycosylated intracellular form of IL-1ra , with seven extra amino acids and without a signal peptide, with activity comparable to that of secreted IL-1ra , has also been described. IL-1ra is capable of binding effectively to IL-1R.sub.I and much less well to IL-1R.sub.II. IL-1R.sub.I, the type I IL-1 receptor, is a receptor that belongs to the immunoglobulin superfamily, composed of an extracellular domain (which has three immunoglobulin-like units bound by disulphide bridges), a transmembrane sequence that anchors the receptor to the cell, and an intracellular domain that is responsible for transmitting the activation signal to the interior of the cell. The other IL-1 receptor, IL-1R.sub.II, is structurally very similar to IL-1R.sub.I in the extracellular and transmembrane part, but possesses practically no intracellular domain and therefore does not seem capable of transmitting the activation signal. It is therefore hypothesized that IL-1R.sub.II does not have the ability to activate the cells and that IL-1R.sub.I is largely responsible for cell activation in response to IL-1 (Arend W.P. J. Clin. Invest. 88: 1445, 1991; Dinarello C.A. & Thompson R.C. Immunol. Today 11: 404, 1991). IL-1R.sub.II is released naturally by the cell membrane, probably through the action of a specific protease, and once it is free in the extracellular space it is able to capture circulating IL-1.beta. and prevent it from interacting with membrane IL-1R.sub.I, so that it functions as an IL-1 inhibitor. However, the actual biological role of IL-1R.sub.II, apart from capture and inhibition of IL-1 when released by the cell in soluble form, has not yet been elucidated definitively and there are data in various systems that suggest possible cell activation that is dependent on IL-1R.sub.II (Boraschi et al. Neuro-Immunology of Fever p. 19, 1992; Luheshi G. et al. Am. J. Physiol. 265: E585, 1993; Kent S. et al. Proc. Natl. Acad. Sci. USA 89: 9117, 1992. IL-1ra does not have IL-1-like biological activity, in that it occupies IL-1R.sub.I without activating the cell, and in occupying the receptor it functions as an antagonist of IL-1 activity. On account of its antagonist activity, IL-1ra has been used successfully in experimental models of inflammation induced by IL-1, by LPS (a bacterial endotoxin) and by live bacteria, to inhibit the inflammatory, toxic and lethal pathologic effects of the treatments (Ohlsson K. et al. Nature 348: 550, 1990; Wakabayashi G. et al. FASEB J. 5: 338, 1991; Alexander H.R. et al. J. Exp. Med. 173: 1029, 1991; Fischer E. et al. J. Clin. Invest. 89: 1551, 1992). However, in view of the extreme potency of IL-1 (which can activate cells by occupying fewer than ten receptors/cell), the doses of IL-1ra necessary to obtain significant therapeutic effects in vivo are extremely high. Trials in humans in septic shock have shown a marginal efficacy of IL-1ra even at extremely high doses (Fischer C. J. et al., J.A.M.A. 271: 1836, 1994). Accordingly, it is particularly useful to be able to modify the structure of IL-1ra so as to increase its capacity for interaction with IL-1R.sub.I and so improve its therapeutic efficacy. An IL-1ra mutant having a glycine instead of asparagine in position 91 has been recently disclosed (Evans R. et al. J. Biol. Chem., 11477, 1995).